1. Field of the Invention
Of the factors impacting the fuel economy of trucks, tractor-trailers, and other long haul vehicles, it is their aerodynamic efficiency that is, at present, most possible to improve. While tractors are relatively aerodynamic, trailers must assume the form of a box, which, other than those devices intended to intake or inhibit the flow of air, represents the least aerodynamic design. The tractor-trailer is subject to substantial drag as a result of the abrupt termination of the trailer walls at its aft, which creates a partial vacuum and causes turbulent airflow. Ideally speaking, the contour of the trailer and its influenced airflow would taper to a single point or terminating edge, as in a teardrop shape.
In spite of this flaw, the rectangular shape of a box is the only sensible design, as a trailer with an aerodynamic contour will reduce the volume of interior cargo space. It is impossible to compensate for this reduction by increasing the overall length of the trailer, which cannot exceed 53′ because of legal restrictions and practical limitations. Furthermore, any modifications to the primitive shape of an ordinary trailer that cut away insignificant portions of its cargo space would have little impact with regard to its aerodynamic efficiency.
Consequently, an additional apparatus attached to the rear of the trailer is the only means of achieving an aerodynamic form. While the standard trailer is limited to 53′ the federal government allows aerodynamic attachments an exclusion from normal length and width determinations. According to §658.16 of the Federal Motor Carrier Safety Administration regulations: “Aerodynamic devices that extend a maximum of 5 feet beyond the rear of the vehicle, provided such devices have neither the strength, rigidity nor mass to damage a vehicle, or injure a passenger in a vehicle, that strikes a trailer so equipped from the rear, and provided also that they do not obscure tail lamps, turn signals, marker lamps, identification lamps, or any other required safety devices, such as hazardous materials placards or conspicuity markings.”
The collapsed size of an attachment, although not subject to a particular statutory restriction, is equally important to consider. The trailer doors to which an aerodynamic apparatus is affixed have been designed to open all the way back to the sidewalls, fitting flat against the trailer to facilitate docking. When flattened, folded, or deflated, any such apparatus must be small enough to fit within the narrow space normally existing between the trailer and its doors. While a wide variety of devices would allow limited use of the doors, only a handful of designs can completely satisfy this condition.
2. Description of the Prior Art
As the most aerodynamic apparatus would assume the tapered contour of the teardop, it is probable that a percentage of prior art should fit this basic profile. None of these designs, however, satisfy the necessary conditions of a working device. For example, the ‘vehicle drag reducer’ of U.S. Pat. No. 4,257,641 shares a similar shape, but this is achieved by replacing the standard doors instead of by mounting an additional apparatus. Not only would this design increase the length of the trailer without increasing cargo capacity, but such curved doors could not close completely, prohibiting temperature control or other advantages associated with conventional cargo containers.
The ‘reduced drag trailer’ of U.S. Pat. No. 6,286,894 and ‘trailer with aerodynamic rear door’ of U.S. Pat. No. 7,699,382 B2, are subject to similar flaws. These devices could not easily be retrofitted to an existing cargo container. On account of the problems associated with major modifications to the main body of a trailer, the majority of patents in the field of this invention propose the addition of an attached apparatus.
While the drag reducing fairing of U.S. Pat. No. 4,458,936 exhibits a shape similar to the present invention, its construction is completely different and its use ultimately impractical. Proposed to be produced from rigid lightweight plastics or metal skin, this attached apparatus is a solid structure and as such could not be collapsed or otherwise undeployed. While it is suggested that this rear fairing could be readily detached, this process would prove impractical, particularly as removal need be regularly performed. Not only would such a device be rigid and uncollapsible, but with a volume as large as 150 cubic feet, its dimensions would make it unmanageable. Since the removal and replacement of any such fairing would be inordinately complicated and time consuming, this type of device must remain attached. As trailer doors are regularly opened and closed, an attached apparatus should be constructed such that it can be collapsed or deployed at will.
A percentage of prior art is designed with this requirement chiefly in mind, but consequently lacks the preferred contour present in other aerodynamic apparatuses. Instead, these devices consist of flat panels extending from the top, bottom, and sidewalls of the trailer, as in the ‘apparatus for reducing drag’ of U.S. Pat. No. 5,498,059 or ‘air drag reducing apparatus’ of U.S. Pat. No. 6,485,087 B1 and U.S. Pat. No. 6,257,654 B1. Whether deployed or collapsed, these panels are always flat, and as such cannot form the preferred contour. While such a shape is easily folded to fit with the door, these panels merely modify the existing shape of the trailer rather than transform it. Such a configuration is insufficient to eliminate the partial vacuum present at the rear of the trailer during transit or its deleterious effects on fuel economy.
Paneled construction could be used to create curved contours, as in the ‘deployable airfoil’ of U.S. Pat. No. 6,666,498. While this configuration creates a more aerodynamic contour than similar devices, it suffers from a fatal flaw. Namely, it is dependent upon panels with a preexisting and persistent curved shape, meaning that they can never fold flat against the doors such that they may be opened completely, which is to say folded against the sides of the trailer, a necessary condition for docking at nearly every facility freight is delivered. The difficulty of docking is compounded by a related deficiency, that these panels could not be folded back onto themselves and as a result extend beyond the length of the swing doors, making their opening and closing cumbersome or hazardous.
The desired shape could also be achieved through the use of air bladders, as in the ‘inflatable drag reducer’ of U.S. Pat. No. 4,741,569, the ‘drag reducing apparatus’ of U.S. Pat. No. 5,823,610 and the ‘vehicle streamlining structure’ of U.S. Pat. No. 6,457,766, but their assembly lacks significant support structures. Due to their insubstantial construction, these devices would be unstable under the highway conditions at which they would otherwise yield their greatest benefit. In such a scenario, an inflatable device is subject not only to aerodynamic turbulence but strong crosswinds that would further destabilize its form and undermine its aerodynamic efficiency. Furthermore, suggested methods for ducting air to the rear of the trailer, in order to sustain the shape of such a device or perform an auxiliary function, are either too complex or compromise its main purpose, which is to reduce drag.
Those devices that collapse rather than deflate are also fraught with difficulties. Many proposed devices would inhibit or prevent outright the normal operation of trailer doors. The ‘aerodynamic drag reducing apparatus’ of U.S. Pat. No. 7,380,868 consists of nested frames that could be extended to create a tapered form. Even when retracted, these frames would restrict the motion of the doors by as much as one third. As the collapsed depth of the device along the edge nearest the hinge must be as little as one inch, this imposes insurmountable limitations to the size of these nested frames. Related U.S. Pat. No. 7,618,086 improves upon its predecessor, but would nevertheless restrict use of the doors. While the accordion type apparatus is more compact, it would even in its collapsed stated prevent the doors from folding flat against the trailer.
As the full range of motion associated with normal use of the trailer doors is necessary for docking, any device which fails to satisfy this requirement is impractical to use and pointless to produce.